"I need a friend"
Someone explained the "na" as a tense marker, present tense. Which made more sense to me.
E.g earlier we learnt that "tutaonana baadaye"=see you later
Breaking "tutaonana" would go something like Tu = We ta (tense marker, future)= will onana = see each other (from verb "kuona"= to see)
I suppose it will make more sense when we learn other tenses
It appears you're confusing this with verbs that take -i in the negative form but -a otherwise.
Verbs usually end with vowels(or all?, not really sure...) and just because it ends with -i does not necessarily mean that if it ends in -i it is negative. It can though.
Negative verbs in present tense:
-Do not have the tense marker (-na-)
-Will take a negative subject pronoun marker instead of a positive one(Negative ones are Si-, hu-, ha-, hatu-, ham-, and hawa-, the non-negative ones are Ni-, U-, A-, Tu-, M-, Wa-)
-If the verb ends with -a in the positive, it will change to -i in the negative(which is where I think the confusion originated from) Example: Ninapenda becomes Sipendi